Natural attenuation of large anthropogenic nitrate loads in a subtropical stream revealed by δ15N and δ18O

•Agricultural creeks can mitigate downstream nitrogen loads and ecosystem impacts.•Creeks can remove up to ~80% of upstream nitrogen load per km of downstream creek.•Recycled sewage water used on farms contributes to nitrogen loads in nearby creeks.•Spikes in creek nitrogen are likely driven by flushing of soil storages during rainfall. Nitrogen pollution in subtropical waters is rapidly increasing due to land-use change, but specific sources, transformations, and attenuation rates remain understudied compared to cooler temperate catchments. Here, we quantify high-resolution nitrate (NO3−-N) loads, sources and natural attenuation in a subtropical creek in Australia over contrasting hydrological conditions. We observed large creek NO3−-N loads (ranging from 44 to 2938 μmol m−2 catchment d−1) exceeding the bottom-up estimates of nitrogen input to the catchment at the most upstream sites. Stable isotope natural abundances (δ15N and δ18O in NO3−-N) and Bayesian analysis revealed that greywater was the dominant source accounting for ~55% of NO3−-N in the upper creek, but fertilisers (~29%) and rainfall (~16%) were also relevant NO3−-N sources. NO3−-N loads at the most downstream site were only 0.2–9.7% of loads at the most upstream site. The resulting NO3−-N attenuation efficiency (mainly via denitrification) was 52–84% of original upstream load per km of creek, depending on hydrological conditions. This large capacity to attenuate NO3−-N during dry and first-flush events exceeds the attenuation found in temperate creeks subject to several decades of pollution in the northern hemisphere. During periods of high water flow and saturated soils, high NO3−-N loads were exported downstream, turning the creek from a natural bioreactor to a system resembling a flow-through pipe. In spite of effective natural nitrogen attenuation providing a valuable ecosystem service, concentrations and loads remained well above expected for natural systems and water quality guidelines. Overall, our results highlight the need for modifying fertiliser use, capturing nitrogen on farms and reducing greywater nitrogen to prevent significant losses to subtropical waterways.